To better protect occupants in an automobile it is necessary to measure the belt force of a person wearing a seatbelt. The data is processed and used during an accident to calculate the optimum triggering of belt tensioners and airbags.
The measuring device should therefore measure belt forces in the range 0–150 Newton with an accuracy of approximately +/−2.5%. Belt forces of up to 10,000 N can occur during standard operation. These must not cause damage to the measuring device or seatbelt lock. During an accident the belt force can rise to 20,000 N. The seatbelt lock and measuring device may, in this case, sustain permanent deformation damage but must not fail to operate properly.
A tried-and-tested principle for measuring forces is to measure displacement on a resilient component on which a force has impinged (spiral spring, torsion spring, coil spring, for example). An overload can easily be intercepted here if, after traveling beyond a path or angle specified in the design, the resilient component strikes a mechanical stop (step, edge).
U.S. Pat. No. 6,230,088 B1 discloses a force-sensing unit located between a seatbelt lock and an anchoring point secured to a vehicle's bodywork. However, this already known force-sensing unit for measuring a belt force is complex in its structure and difficult to install.
The invention, on the other hand, provides a simply structured and easily installable force-sensing unit that exhibits a sufficient degree of accuracy in measuring low-level forces.
In one embodiment of the invention, a force-sensing unit is provided which can be produced using standard components and is easy to install. A torsion spring is a simple component which supplies well reproducible values when bent within the elastic area.
In another embodiment of the invention, the torsion spring is formed by two arms advantageously designed to be an integral part of the anchor, can be manufactured economically. Furthermore, it is easy to detect after an accident whether the sensor has an anticipated zero point and consequently does not need replacing, which will save on repair costs. If the zero point has been shifted, meaning that a value exceeding 5 N, for example, is measured without the impact of a force, the force-sensing unit needs to be replaced.
In still another embodiment of the invention, a force-sensing unit is provided which can be easily secured to a seatbelt lock. The force-sensing unit can be manufactured separately and integrated later by the seatbelt lock manufacturer. This thus also makes it possible to subject the sensor to partial (gain and temperature) or complete (offset, gain, temperature) calibration prior to installation on the seatbelt lock or prior to delivery to the seatbelt lock manufacturer, which would simplify the manufacturing process as the seatbelt lock manufacturer will require minimal/no electronic calibration equipment.
Another advantage of the invention lies in the force-sensing unit's being easy to retrofit, with the possibility of securing it by simple means to a conventional seatbelt lock.
The force-sensing unit according to the invention can be used, for example, for optimum triggering of a belt tensioner and/or airbag.
The same reference numbers are used below for the same components in FIGS. 1 to 4.